Associations between body size and visual impairment of first-year university students in Chongqing: A cross-sectional study

The relationship between body size and visual impairment (VI) presents a controversial topic in the health sciences. This study aims to evaluate and clarify the potential associations between these 2 variables. We conducted a cross-sectional study on first-year students enrolled in 2022 at the Southwest University of Political Science & Law. The students underwent a series of physical examinations and visual acuity tests. Visual impairment was classified into 3 categories: mild, moderate, or severe. We used logistic regression analysis to examine the association between body size and VI. Our findings indicated a high prevalence of VI among first-year university students; more than 80% of them were affected. In bivariate analysis, height and weight were negatively related to the presence of VI. However, BMI (body mass index) was not related to VI. By adjusting all available confounders, no associations between BMI (OR = 1.002, 95% CI = 0.974–1.032, P = .877), height (OR = 0.998, 95% CI = 0.967–1.010, P = .298), weight (OR = 0.999, 95% CI = 989–1.009, P = .860), and mild-severe VI were found in females. For males, the ORs were 0.988 (95% CI = 0.955–1.021, P = .459), 0.980 (95% CI = 0.954–1.006, P = .135), and 0.995 (95% CI = 0.985–1.004, P = .285) for BMI, height, and weight, respectively. Among young adults demonstrating high academic performance in high schools, the cessation of physical growth, combined with potential eye strain resulting from overuse, may mitigate any previously observed positive associations between physical status and VI in younger children.


Introduction
Globally, an estimated 100 million people suffer from visual impairment (VI), with prevalence rates alarmingly higher among younger demographics. [1,2]The World Health Organization has identified VI as a priority area under Vision 2020, particularly focusing on school-aged students in developing countries. [3]arious studies have been conducted to identify potential risk factors for VI, including lifestyle changes, near-sighted work, insufficient outdoor activities, academic pressure, and a family history of high myopia. [4,5]ody size has previously been linked to VI, as taller and overweight individuals may have larger eyes and longer axial lengths (AL), potentially leading to myopia-associated VI. [6] For instance, a study conducted in Central China on students aged 6 to 21 found a positive association between overweight or obesity and VI after adjusting for age and sex. [6]wever, the relationship between VI and body stature remains inconsistent across studies. [7,8]In South Korea, a survey of individuals aged 5 to 18 revealed that those in higher quintiles of height and weight showed a higher prevalence of myopia. [9]Similarly, taller Chinese children in Singapore typically had lower vision and longer axial lengths. [10]In contrast, taller Chinese adults (aged 41-60) in Singapore had eyes with longer axial lengths without a high prevalence of VI. [7] Interestingly, most previous studies found the correlation between body size and VI prevalence only in children and adolescents, not in adults. [9]hen, we predicted that once body size reached a mature level, the impact of physical state on myopia and VI during growth may diminish.However, the exact age at which this association between body size and VI vanished remained unclear.In this study, we aim to investigate whether the relationship between JZ and RL contributed equally to this work.
Informed consent was obtained from all subjects involved in this study.

The authors have no funding and conflicts of interest to disclose.
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

The original data collection was conducted according to the guidelines of the Declaration of Helsinki and approved by Ethics Committee of the Second Affiliated Hospital of Chongqing Medical University (No. CAF52704054B).
body size and VI in a young adult with high academic performance still exists.

Subjects and methods
This study was conducted with a cross-sectional design.All 2022 first-year students at Southwest University of Political Science & Law, a prestigious institution in China where only outstanding students across China have chances to be enrolled, were invited.From them, 201 students who had myopia correction surgery were excluded.
Ethical approval (No.CAF52704054B) was obtained from the Ethics Committee of Chongqing Medical University, according to the principles of the Declaration of Helsinki.Written informed consent was obtained directly from participants.Informed consent was obtained from all subjects involved.

Visual acuity measurement
Two qualified ophthalmologists measured visual acuity (VA) as uncorrected visual acuity (logMAR chart) at a distance of 5 meters.VI considered an uncorrected VA of 20/40 (or less than 20/40) in either eye.VI was classified as mild (between 20/40 and 20/70), moderate (between 20/70 and 20/200), and severe (between 20/200 and 20/400) visual impairment. [11]ecause the right eye and the left eye are highly correlated in visual acuity, only the VA of the right eye was used in the analysis as reported before. [12]

Physical examination and body index definition
Weight (kilograms) and height (centimeters) were measured on a human body scale.body mass index (BMI) was calculated by dividing weight by height (kg/m 2 ) and categorized into underweight, normal, overweight, and obese with < 18.5 kg/m 2 , <24 kg/m 2 , <28 kg/m 2 , and ≥ 28 kg/m 2 cutoffs; respectively. [13]3.Questionnaire survey Demographic, sociological, and behavioral information related to eyesight was collected by a self-administrated, web-based questionnaire at the site.The survey was conducted by 8 groups of researchers, including 2 qualified ophthalmologists, 3 ocular assistants, 2 enumerators, and a professional statistical analyst.

Statistical analyses
Two ophthalmologists checked the data, independently.Continuous variables were compared with ANOVA, and the Chi-square test or Fisher's exact was used to compare categorical variables.The relation between VI and BMI was evaluated with a multiple logistic regression model.P < .05 was considered statistically significant.Analyses were performed using the statistical analysis software SPSS 16.0.

General characteristics of the study participants
Of the 3267 eligible students, 36 participants were excluded due to missing data, and finally, a total of 3231 students (98.9%) were included in the analysis.The characteristics of students aged 16  the prevalence of vision impairment was a little smaller that of females in each group.

The BMI and VI
The BMI categories in Table 2 compare the prevalence of the visually impaired (the right eye).The prevalence of severe vision impairment was 22.4%, 21.6%, 15.4%, and 21.4% in the underweight, normal, overweight, and obese participants, respectively.The prevalence of VI in each group has no significant difference (P value = .071).

Bivariate logistic regression analysis of factors of VI
In the bivariate logistic analysis, BMI was not related to VI for both mild-severe VI and moderate-severe VI.Height and weight were negatively related to VI.Besides the body size, other factors, such as being female, father or mother having myopia, late owning a smartphone, occasional or disliked exercise, few breaks when learning, and a low education level of the father or mother, were significantly related to the presence of VI.See Table 3.

Discussion
Our study found no relationship between body size and VI prevalence in first-year university students.These results were consistent with some previous research.A study reported [14] that is no association among height, weight, or BMI with myopia in 19-year-old males.Furthermore, an Israeli study on participants aged 17 to 19 reported that myopia is not related to body stature, [15] and another study showed no significant relation between height and myopia. [16]However, many reports showed that body stature was related to myopia or VI prevalence, especially in younger students. [17,18]n the process of human development from infants to adults, refractive status is determined by balancing the refractive power of the cornea and lens with the AL of the eyes. [19,20]A human was born with physiologic hyperopia (+2.50 to +3.00 D), [21,22] called hyperopia reserve.As the body grows, human hyperopia refractions go towards emmetropia.This process is called emmetropization and is usually finished at 15 years old [23] with the emmetropia refractive status (−0.50 to +0.50 D). [24] Ocular dimension change was concomitant with physical development in children. [25]Therefore, myopia may occur due to growth spurts in childhood because of prolonged AL. [26,27] Height being positively associated with AL has been confirmed in many studies. [10,17,28]It can be explained in previous reports why height and VI prevalence were significantly related in younger children.However, when body development stops at age 16 to 18, near-sight work and overusing eyes can keep AL elongation going through the regulation of extraocular and ciliary muscles, which can continue to attenuate the influence of physical development on VI.Our study confirmed our hypothesis that physical status might not be related to VI in young adults.
Subjects in our study have high academic performance, which requires long-time, near-sight work.Previous studies have found a definite correlation between higher educational levels and VI. [9,29]Also, a high prevalence of myopia was found in urban areas than in rural regions, [9] which indicated that near work and less time outdoors are the major risk factors for VI.Therefore, it is less likely for us to observe the influence of physical status on VI in first-year students at a prestigious university.
Interestingly, reminders for the eye-using break are likely protective factors.However, in our multiple logistic regression, it was negatively related to VI.It might be the reverse causality because severe VI may draw parents more attention to remind their children.

Strengthening and limitations
One of the strengths of this study was the high homogeneity of the subjects, which excludes many confounding factors such as age, education level, and academic performance that may exist in other studies.Furthermore, we investigated the relationships between physical and mild and moderate VI, respectively, which confirmed our results in different conditions.Besides, we adjusted the relationship with various lifestyle factors, making our results reliable.
Unfortunately, we did not measure AL at this time; that was a limitation of our study, so we cannot confirm if there is a relationship between AL and body size, which can explain the relationship between VI and body stature.Only one university was included in this study, which may not be generalized to those of the same age.Furthermore, some other confounding factors, such as residence place, sleep duration, studying hours, leisure activities, and myopic correction, were not collected and analyzed, which can bias our results.Despite these limitations, our reports provide valuable references regarding the relationship between VI and body status in young adults.

Conclusion
In conclusion, the relationship between body size and VI confirmed in children disappeared in young adults with high academic achievements.Our study found no relationship between body size, such as weight, height, and BMI, and VI prevalence in first-year university students.The cessation of physical growth, combined with potential eye strain resulting from overuse, may mitigate any previously observed positive associations between physical status and VI in children.However, the results showed activities like doing outdoor exercise or having breaks for using the eyes when learning are protective factors against VI, and encouraging these protective activities will be beneficial in schools or in communities.More studies are warranted to further confirm the relationship between body size and VI in young adults.